An evaporated fuel treatment system prevents the dissipation of evaporated fuel produced in a fuel tank to the atmosphere. The evaporated fuel in the fuel tank is introduced into a canister having an adsorbing material and is temporarily adsorbed by the adsorbing material. The evaporated fuel adsorbed by the adsorbing material is desorbed by a negative pressure developed in an intake pipe when an internal combustion engine is operated and is purged to the intake pipe of the internal combustion engine through a purge passage. When the evaporated fuel is desorbed from the adsorbing material in this manner, the adsorption capacity of the adsorbing material is recovered.
When the evaporated fuel is purged, the flow rate of an air-fuel mixture containing the evaporated fuel is controlled by a purge control valve disposed in the purge passage. However, in order to control the quantity of evaporated fuel actually purged to the intake pipe to an appropriate air-fuel ratio by the purge control valve, it is important to measure the concentration of the evaporated fuel in the air-fuel mixture flowing in the purge passage with high accuracy.
JP-5-18326A shows a system in which mass flow meters are disposed in a purge passage and an atmosphere passage branched from the purge passage. The concentration of evaporated fuel in an air-fuel mixture supplied to an intake pipe of an internal combustion engine from the purge passage is detected on the basis of the output values of the two mass flow meters.
However, the flowmeter is disposed in the purge passage in this system, so the concentration of the evaporated fuel cannot be detected unless the air-fuel mixture containing the evaporated fuel is purged and flows in the purge passage. In order to reflect the detected concentration of the evaporated fuel in the control of an air-fuel ratio, it is necessary to measure the concentration of evaporated fuel before the purged evaporated fuel reaches the injector position. It is necessary to correct the amount of fuel to be injected from the injector based on the measured concentration of evaporated fuel.
However, in the case of an engine having a small intake pipe volume or in an operation range of a high flow velocity of intake air, the time required for purged evaporated fuel to reach the injection position is shorter than the time required for completing the measurement of an evaporated fuel concentration. Thus, it may be impossible to reflect a measured evaporated fuel concentration. Therefore, an engine structure including the layout of pipes and the operation range of starting purge may be restricted.
It can be thought as means for solving the above problems that an air-fuel mixture containing air and evaporated fuel is flowed through a restriction to detect the amount of change in the pressure of air caused by the restriction and the amount of change in the pressure of the air-fuel mixture caused by the restriction. The flow rate of the air-fuel mixture introduced into an intake pipe of an internal combustion engine from a canister is controlled on the basis of the amounts of change in the two amounts of change in pressure.
The amount of change in the pressure caused by the restriction is changed by the density of fluid flowing through the restriction, as is known as Bernoulli's theorem. The amount of change in the pressure when gas containing 0% evaporated fuel (that is air) of a reference gas is flowed through a restriction is compared with the amount of change in the pressure when an air-fuel mixture containing evaporated fuel is flowed through the restriction. A difference in density between both gases can be detected. This difference in density corresponds to the evaporated fuel concentration of the air-fuel mixture. Thus, the evaporated fuel concentration of the air-fuel mixture can be known on the basis of the two amounts of change in pressure (refer to U.S. Pat. No. 6,971,375B2).
When an evaporated fuel concentration is computed on the basis of the amount of change in pressure caused by a restriction, it is preferable that the amount of change in pressure caused by the restriction is changed only by the evaporated fuel concentration of the air-fuel mixture and is not changed by other conditions.
However, the fuel tank always communicates with the canister and hence the canister communicates with the restriction in a state in which the amount of change in pressure caused by the restriction is measured. Thus, when pressure in the fuel tank is changed due to a swing of fuel in the fuel tank, the variation in pressure propagates to the restriction. This variation in pressure is detected by a pressure sensor. For this reason, there is a possibility that when fuel swings, the amount of change in pressure caused by the restriction is changed. Moreover, when the fuel tank communicates with the restriction also in a state in which the amount of change in pressure of air, caused by the restriction, is measured, there is a possibility that the amount of change in the pressure of air, caused by the restriction, is changed by the swing of fuel. When the amount of change in the pressure of the air-fuel mixture or air, caused by the restriction, is changed by the swing of fuel, the accuracy of controlling the flow rate of the air-fuel mixture is lowered to increase the amount of deviation of the air-fuel ratio from the stoichiometric air-fuel ratio.
The present invention has been accomplished in view of these circumstances. An object of the present invention is to provide an evaporated fuel treatment system that can control the flow rate of an air-fuel mixture introduced into an intake pipe with higher accuracy.